(672d) Developing Reactive Models of Polymer Pyrolysis Using Physical Experiments and Reactive Molecular Dynamics

A combination of experimental and computational techniques is being used together to develop molecularly accurate models of polymer pyrolysis and its kinetics. Understanding the kinetics and mechanisms of polymer pyrolysis has far-reaching applications from plastic recycling to solid rocket fuels. In this work, we have studied polyoxymethylene (POM) and poly(methyl methacrylate) (PMMA) using a variety of experimental and computational techniques to provide insights into the mechanisms by which polymers breakdown during polymer pyrolysis.

Experiments were carried out using thermogravimetric analysis / differential scanning calorimetry (TGA/DSC), flash pyrolysis, and gas chromatography (GC) and two-dimensional GC with time-of-flight mass spectrometry (GCxGC/ToFMS), which give overall kinetics of mass loss, heats of melting and reaction, and compositions of gas and condensable products. This combination of rate data and products is crucial to proposing pathways and establishing mechanisms. However, except by DSC, there is no detection of reactions that do not produce volatile products, such as scission of polymer chains into nonvolatile products.

Reactive Molecular Dynamics (RMD) can fill in the knowledge gaps present in physical experiments. RMD can capture both the bond breaking and the physical strains that lead to bond scission. Our previous development of RMD through the RxnMD code and RMDff force field has demonstrated the ability to predict bond-breaking energies and pyrolysis products of high-density polyethylene [1]. Our present work extends that approach to investigate POM and PMMA pyrolysis using LAMMPS/ReaxFF.

References

[1] K. D. Smith, M. Bruns, S. I. Stoliarov, M. R. Nyden, O. A. Ezekoye, and P. R. Westmoreland, “Assessing the effect of molecular weight on the kinetics of backbone scission reactions in polyethylene using Reactive Molecular Dynamics,” Polymer, vol. 52, no. 14, pp. 3104–3111, Jun. 2011, doi: 10.1016/j.polymer.2011.04.035.